If you remove the files in the world\players
and world\stats
folder, the server will forget that they were there. Files bear the user name of the player, so you can delete selectively. If this is a bukkit server, there is a players
folder in each world. Files are named with the ID of the player, so you can delete selectively. To get the ID of a player, open https://api.mojang.com/users/profiles/minecraft/(playername)
(substitute (playername)
with the actual player name)